Satellite communications system having multiple downlink beams powered by pooled transmitters

ABSTRACT

A satellite communications system employs separate subsystems for providing broadcast and point-to-point two-way communications using the same assigned frequency band. The broadcast and point-to-point subsystems employ an integrated satellite antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones in a manner which permits multiple reuse of the entire band of assigned frequencies. A single pool of solid state transmitters allows rain disadvantaged users to be assigned higher than normal power at minimum cost. The intermodulation products of the transmitters are geographically dispersed.

TECHNICAL FIELD

The present invention broadly relates to satellite communicationsystems, especially of the type employing a satellite placed ingeosynchronous orbit so as to form a communication link between manysmall aperture terminals on the earth. More particularly, the inventioninvolves a communications satellite having multiple transmitters and theability to simultaneously provide thousands of narrow, high gain antennabeams while drawing on the satellite's entire pool of transmitter powerso that disadvantaged users may be accommodated with higher power,without reducing the satellite's overall channel capacity.

BACKGROUND ART

In a typical satellite system, either an areawide (e.g. nationwide)antenna beam or narrow zone beams are employed for communications. In Kuband, the satellite communication band most suitable for two-way serviceto very small terminals, the attenuation of the signals by rain is animportant design consideration. The rain attenuation is overcome on thedownlink by using higher satellite transmission power per channel thanwould be necessary for clear weather service, typically four times asmuch. This approach to accommodation of rain attenuation thereforeresults in more expensive satellites having fewer available channels.

Typically, the geographic region covered by the satellite is dividedinto zones with one downlink antenna beam being dedicated to each zone.When zone downlink beams are employed, usually each transmitter isassociated with each zone and cannot be simultaneously used with anyother zone. Although the use of zone beams is advantageous in that zonebeams have high antenna gain, these systems do not sufficientlyaccommodate disadvantaged downlink users. If a downlink user is locatedin an area experiencing heavy rainfall, in order to compensate for therain attenuation, the uplink signal has to be made more powerful. Sinceeach of the zone beams has a limited amount of power associated with it(i.e. the amount of power from its transmitter), the extra power neededto compensate for rain comes from decreasing the amount of powerprovided to the rest of the downlink users in that particular zone.Since the unattenuated users in the zone receive less power, the numberof available channels in that particular zone decreases because there isnot enough power to supply all the users.

Conversely, systems employing one nationwide antenna beam have anationwide pool of power available to them because the nation is servedby all of the transmitter. To compensate for rain attenuation, theuplink power can be increased without overtaxing the other users locatedthroughout the nation. This is because the area over which heavy rainoccurs at any instance, is small compared to the area of the entirenation. Hence the average penalty to the impaired users decreases sincetheir loss is spread out over a larger number of unimpaired users.Despite having a pool of power available, a nationwide beam is not idealfor two-way communications because a wide antenna beam means low antennagain, and for communication to very small earth terminals, a high gainantenna beam is highly desirable.

SUMMARY OF THE INVENTION

The satellite communications system of the present invention combinesthe advantages of a narrow beam, i.e. a high downlink EIRP, withavailability of a nationwide pool of transmitter power, thus making iteasy to overcome signal degradation due to factors such as rain. Thepresent system makes it feasible to overcome rain attenuation withoutreducing the statellite's overall channel capacity because theadditional power required to compensate for rain attenuation comes froma large pool of transmitter power which serves the entire nation. Thus,the average penalty on unimpaired users is minimized because thereduction in available power is averaged out over thousands of users.

According to one aspect of the invention, a method is provided forcommunicatively interconnecting any of a plurality terminal sites withinan area on the earth using a communications satellite. A plurality ofradio frequency uplink beams, each carrying a receive signal, aretransmitted from uplink terminals sites in the area to a satellite,which is preferably placed in geosynchronous orbit above the earth. Theuplink beams are received at the satellite and the receive signals areconverted to corresponding transmit signals destined to be transmittedto downlink terminal sites in the area. All of the transmit signals arecollectively amplified using a plurality of amplifiers at the satellitesuch that each of the transmit signals is amplified collectively by allof the amplifiers. The satellite transmits to the area a plurality ofdownlink beams respectively covering portions of the area, wherein eachof the downlink beams carries one of the transmit signals to be receivedby a downlink terminal site in a corresponding portion of the area. Thepower of one or more of the downlink beams is increased in order toovercome the attenuating effects of rain by increasing the power of theuplink beam carrying the corresponding receive signal.

According to another aspect of the invention, an apparatus is providedfor communicatively interconnecting any of a plurality of terminal siteswithin an area on the earth. The apparatus includes a satellitepreferably positioned in a geosynchronous earth orbit. The satellitecarries means for respectively receiving from uplink terminal sites inthe area a plurality of uplink radio frequency beams each carrying areceive signal. Means carried by the satellite are provided forconverting the receive signals into transmit signals, each of whichincludes a plurality of corresponding subsignals destined to be receivedat downlink terminals sites in the area, where the conversion isperformed by changing the frequencies of the receive signals. Aplurality of amplifiers carried by the satellite collectively amplifyall of the transmit signals such that each of the transmit signals isamplified by all of the amplifiers. The satellite includes beam-formingmeans for forming a plurality of downlink radio frequency beams eachcarrying one of the transmit subsignals destined to be received by oneof the downlink terminal sites and covering only a portion of the area.Means are also provided on the satellite for respectively transmittingthe downlink beams to corresponding portions of the area. Each of thetransmit subsignals is a frequency division multiplex signal, and eachof the downlink beams is frequency addressable. The transmitting meansincludes an array of antenna elements for radiating electromagneticenergy, wherein the antenna array has a plurality of input transmitarray elements respectively coupled with the amplifiers of the transmitsignals. The power of the receive signals in each of the uplink beams isessentially proportional to the power of the corresponding subsignal ineach of the downlink beams.

It is a primary object of the present invention to provide a satellitecommunications system providing a multiplicity of narrow, high-gaindownlink beams collectively powered by a pool of individual transmittersin order to overcome localized signal degradation to rain or the likewithout reducing the channel capacity of the system.

Another object of the present invention is to provide a system asdescribed above which allows accommodation of disadvantaged downlinkuser sites without the need for substantially reducing the power ofdownlink signals transmitted to non-disadvantaged downlink users.

A further object of the present invention is to provide a system asdescribed above having means for forming downlink transmit signals in amanner which allows all of the transmit signals to be amplified by a setof pooled amplifiers in a manner such that each transmit signal isamplified by all of the amplifiers.

These, and further objects and advantages of the present invention, willbe made clear or will become apparent during the course of the followingdescription of a preferred embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a perspective view of a communications satellite, showing theantenna subsystems;

FIG. 2 is a top plan view of the antenna subsystems shown in FIG. 1;

FIG. 3 is a sectional view taken along the line 3--3 in FIG. 2;

FIG. 4 is a sectional view taken along the line 4--4 in FIG. 2;

FIG. 5 is a view of the United States and depicts multiple, contiguousreceive zones covered by the satellite of the present invention, theprimary areas of coverage being indicated in crosshatching and the areasof contention being indicated by a dimpled pattern;

FIG. 6 is a block diagram of the communication electronics for thecommunications satellite;

FIG. 7 is a schematic diagram of a coupling network which interconnectsthe point-to-point receive feed horns with the inputs to thecommunications electronics shown in FIG. 6;

FIG. 8 is a reference table of the interconnect channels employed toconnect the receive and transmit zones for the point-to-point system;

FIG. 9 is a diagrammatic view of the United States depicting multiplecontiguous transmit zones covered by the satellite and the geographicdistribution of the interconnected channels for each zone, across theUnited States;

FIG. 9A is a graph showing the variation in gain of the transmit antennabeam for each zone in the point-to-point system in relation to thedistance from the center of the beam in the east-west direction;

FIG. 9B is a graph similar to FIG. 9A but showing the variation in gainin the north-south direction;

FIG. 10 is a detailed schematic diagram of the filter interconnectionmatrix employed in the point-to-point system;

FIG. 11 is a detailed, plan view of the beam-forming network employed inthe point-to-point system;

FIG. 12 is an enlarged, fragmentary view of a portion of thebeam-forming network shown in FIG. 11;

FIG. 13 is a front elevational view of the transmit array for thepoint-to-point system, the horizontal slots in each transmit element notbeing shown for sake of simplicity;

FIG. 14 is a side view of the transmit element of the array shown inFIG. 13 and depicting a corporate feed network for the element;

FIG. 15 is a front, perspective view of one of the transmit elementsemployed in the transmit array of FIG. 13;

FIG. 16 is a front view of the receive feed horns for the point-to-pointsystem; and

FIG. 17 is a diagrammatic view showing the relationship between atransmitted wave and a portion of the transmit feed array for thepoint-to-point system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1-4, a communications satellite 10 is depictedwhich is placed in geosynchronous orbit above the earth's surface. Thesatellite's antenna system, which will be described in more detailbelow, will typically be mounted on an earth-oriented platform so thatthe antenna system maintains a constant orientation with respect to theearth.

The satellite 10 is of a hybrid communications-type satellite whichprovides two different types of communication services in a particularfrequency band, for example, the fixed satellite service Ku band. Onetype of communication service, referred to hereinafter as point-to-pointservice, provides two-way communications between very small apertureantenna terminals of relatively narrow band voice and data signals.Through the use of frequency division multiple access (FDMA) and reuseof the assigned frequency spectrum, tens of thousands of suchcommunication channels are accommodated simultaneously on a singlelinear polarization. The other type of communication service provided bythe satellite 10 is a broadcast service, and it is carried on the otherlinear polarization. The broadcast service is primarily used for one-waydistribution of video and data throughout the geographic territoryserved by the satellite 10. As such, the transmit antenna beam coversthe entire geographic territory. For illustrative purposes throughoutthis description, it will be assumed that the geographic area to beserviced by both the point-to-point and broadcast services will be theUnited States. Accordingly, the broadcast service will be referred tohereinafter as CONUS (Continental United States).

The antenna system of the satellite 10 includes a conventional omniantenna 13 and two antenna subsystems for respectively servicing thepoint-to-point and CONUS systems. The point-to-point antenna subsystemprovides a two-way communication link to interconnect earth stations fortwo-way communications. The CONUS antenna system functions as atransponder to broadcast, over a wide pattern covering the entire UnitedStates, signals received by one or more particular locations on earth.The point-to-point transmit signal and the CONUS receive signal arevertically polarized. The CONUS transmit and point-to-point receivesignals are horizontally polarized. The antenna system includes a largereflector assembly 12 comprising two reflectors 12a, 12b. The tworeflectors 12a, 12b are rotated relative to each other about a commonaxis and intersect at their midpoints. The reflector 12a is horizontallypolarized and operates with horizontally polarized signals, while thereflector 12b is vertically polarized and therefore operates withvertically polarized signals. Consequently, each of the reflectors 12a,12b reflects signals which the other reflector 12a, 12b transmits.

A frequency selective screen 18 is provided which includes two halves orsections 18a, 18b and is mounted on a support 30 such that the screenhalves 18a, 18b are disposed on opposite sides of a centerline passingdiametrically through the satellite 10, as best seen in FIG. 2. Thefrequency selective screen 18 functions as a diplexer for separatingdifferent bands of frequencies and may comprise an array of discrete,electrically conductive elements formed of any suitable material, suchas copper. Any of various types of known frequency selective screens maybe employed in this antenna system. However, one suitable frequencyselective screen, exhibiting sharp transition characteristics andcapable of separating two frequency bands which are relatively close toeach other, is described in U.S. Patent Application Ser. No. 896534,filed Aug. 14, 1986 and assigned to Hughes Aircraft Company. Thefrequency selective screen 18 effectively separates the transmitted andreceived signals for both the CONUS and point-to-point subsystems. Itmay be appreciated that the two halves 18a, 18b of the screen 18 arerespectively adapted to separate individual signals which arehorizontally and vertically polarized.

The CONUS subsystem, which serves the entire country with a single beam,has, in this example, eight conventional transponders each having a highpower traveling wave tube amplifier as its transmitter 82 (see FIG. 6).The CONUS receive antenna uses vertical polarization, sharing thevertically polarized reflector 12b with the point-to-point transmissionsystem. CONUS receive signals pass through the frequency selectivescreen half 18b and are focused on the receive feed horns 14 located atthe focal plane 28 of reflector 12b. The antenna pattern so formed isshaped to cover CONUS. The CONUS transmit antenna employs horizontalpolarization, and shares reflector 12a with the point-to-point receivesystem. Signals radiating from the transmit feeds 24 are reflected bythe horizontally polarized frequency selective screen 18a to reflector12a whose secondary pattern is shaped to cover CONUS.

The point-to-point subsystem broadly includes a transmit array 20, asubreflector 22, and receive feed horns 16. The transmit array 20, whichwill be described later in more detail, is mounted on the support 30,immediately beneath the screen 18. The subreflector 22 is mountedforward of the transmit array 20 and slightly below the screen 18. Thesignal emanating from the transmit array 20 is reflected by thesubreflector 22 onto one half 18b of the screen 18. The subreflector 22in conjunction with the main reflector 12 functions to effectivelymagnify and enlarge the pattern of the signal emanating from thetransmit array 20. The signal reflected from the subreflector 22 is, inturn, reflected by one half 18b of the screen 18 onto the largereflector 12b, which in turn reflects the point-to-point signal to theearth. Through this arrangement, the performance of a large aperturephase array is achieved. The receive feed horns 16 are positioned in thefocal plane 26 of the reflector 12a. It consists of four main horns 50,54, 58, 62 and three auxiliary horns 52, 56, 60 as shown in FIG. 16.

Referring now also to FIGS. 13-15, the transmit array 20 comprises aplurality, for example forty, transmit waveguide elements 106 disposedin side-by-side relationship to form an array, as shown in FIG. 13. Eachof the transmit waveguide elements 106 includes a plurality, for exampletwenty-six, of horizontal, vertically spaced slots 108 therein whichresult in the generation of a vertically polarized signal. As shown inFIG. 14, the transmit array 20 is fed with a transmit signal by means ofa corporate feed network, generally indicated by the numeral 110 whichexcites the array element in four places 114. The purpose of thecorporate feed network 110 is to provide a broadband match to thetransmit waveguide element 106. Signals input to the waveguide opening112 excite the array slots 108 so that the slot excitation is designedto give a flat pattern in the north-south direction.

Attention is now directed to FIG. 5 which depicts a generallyrectangular beam coverage provided by the horizontally polarizedpoint-to-point receive system. In this particular example, the areaserviced by the point-to-point system is the continental United States.The point-to-point receive system comprises four beams R1, R2, R3, R4respectively emanating from the four uplink zones 32, 34, 36, 38 to thesatellite, wherein each of the beams R1-R4 consists of a plurality ofindividual uplink beams originating from individual sites in each zone32, 34, 36, 38 and carries an individual signal from that site. Theuplink beam signals from the individual sites are arranged into aplurality of channels for each zone. For example, zone 32 may include aplurality, e.g. sixteen 27 MHz channels with each of such channelscarrying hundreds of individual beam signals from corresponding uplinksites in zone 32.

The signal strength for each of the four beam pattern contours,respectively designated by numerals 32, 34, 36 and 38, are approximately3 dB down from peaks of their respective beams. The antenna beams havebeen designed to achieve sufficient isolation between them to makefeasible in the cross-hatched regions 39, 41, 43, 45 reuse of thefrequency spectrum four times. In the dotted regions 40, 42, and 44, theisolation is insufficient to distinguish between signals of the samefrequency originating in adjacent zones. Each signal originating inthese regions will generate two downlink signals, one intended and oneextraneous. The generation of extraneous signals in these areas will bediscussed later in more detail.

It may be readily appreciated from FIG. 5 that the four zones covered bybeams 32, 34, 36, 38 are unequal in width. The East Coast zone coveredby beam 32 extends approximately 1.2 degrees; the Central zone coveredby beam 34 extends approximately 1.2 degrees; the Midwest zone coveredby beam pattern 36 extends approximately 2.0 degrees, and; the WestCoast zone covered by beam pattern 38 extends approximately 2.0 degrees.The width of each of the four receive zones 32, 34, 36 and 38 isdetermined by the number of terminals and thus the population density inthe various regions of the country. Thus, beam pattern 32 is relativelynarrow to accommodate the relatively high population density in theEastern part of the United States while beam pattern 36 is relativelywide due to the relatively low population density in the Mountainstates. Since each zone utilizes the entire frequency spectrum, zonewidths are narrower in regions where the population density is high, toaccommodate the greater demand for channel usage.

As shown in FIG. 9, the point-to-point transmit system comprises fourbeams T1, T2, T3, T4 respectively covering the four transmit zones 31,33, 35, 37, wherein each of the beams T1-T4 consists of a plurality ofindividual downlink beams destined for the individual downlink sites ineach zone 31, 33, 35, 37 and carries an individual signal to that site.The downlink beam signals, destined to be received at the individualdownlink sites, are arranged into a plurality of channels for each zone.For example, zone 31 may include a plurality, e.g. sixteen 27 MHzchannels with each of such channels carrying hundreds of individual beamsignals to corresponding downlink sites in zone 32.

The use of multiple downlink zones and downlink zones of unequal widthsassist in causing the intermodulation products, generated by thelater-discussed solid state power amplifiers, to be geographicallydispersed in a manner which prevents most of these products from beingreceived at the ground terminals. The net effect is that the amplifiersmay be operated more efficiently because the system can tolerate moreintermodulation products. Although the widths of the transmit zones 31,33, 35, 37 are nearly the same as those of the receive zones R1, R2, R3,R4, small differences between the two sets have been found to optimizethe capacity of the system.

The half power beam width of the individual transmit beams 29 issubstantially narrower than that of the transmit zones 31, 33, 35, 37.This results in the desirable high gain, and avoids the zones ofcontention 40, 42, 44 characteristic of the receive zone arrangement.These individual beams 29 must be steered within the zones in order tomaximize the downlink EIRP in the directions of the individualdestination terminals. The transmit point-to-point frequency addressablenarrow beams 29 are generated by an array 20 whose apparent size ismagnified by two confocal parabolas comprising a main reflector 12b anda subreflector 22. The east-west direction of each beam 29 is determinedby the phase progression of its signal along the array 106 of transmitelements 20 (FIGS. 13 and 15). This phase progression is established bya later-discussed beam-forming network 98 and is a function of thesignal frequency. Each of the transmit array elements 20 is driven by alater-discussed solid state power amplifier. The power delivered to thearray elements 106 is not uniform but is instead tapered with the edgeelements being more than 10 dB down. Tapering of the beams 29 isachieved by adjusting the transmit gain according to the position of thetransmit array elements 20. The excitation pattern determines thecharacteristics of the transmit secondary pattern, shown in FIG. 9A.Referring to FIG. 9, the closest spacing between transmit zones 31, 33,35, 37 occurs between zones 31 and 33 and is approximately 1.2 degrees.This means that a signal addressed to zone 33 using a particularfrequency would interfere with a signal using the same frequency in zone31 with its side lobe 1.2 degrees from its beam center. However, theindividual transmit gains have been adjusted to provide low side lobelevels thereby permitting frequency reuse adjacent zone. Referring toFIG. 9A, it is seen that the side lobe level at this angle off beamcenter is more than 30 dB down, so that such interference will benegligibly small. The same frequency uses in zones 35 and 37 are furtherremoved in angle, hence the side lobe interference in those zones iseven smaller.

FIG. 9B is an illustration of the transmit beam pattern in thenorth-south direction. The twenty six slots 108 in each of the transmitwaveguide elements 106 are excited in a manner which creates a nearlyflat north-south pattern, extending over the covered range of plus andminus 1.4 degrees from the north-south boresight direction.

Both the point-to-point and CONUS systems may utilize the same uplinkand downlink frequency bands, with the point-to-point system usinghorizontal polarization for its uplink polarization, and the CONUSsystem using vertical polarization, as previously mentioned. Forexample, both services may, simultaneously, utilize the entire 500 MHzuplink frequency band between 14 and 14.5 GHz, as well as the entire 500MHz downlink frequency band between 11.7 and 12.2 GHz. Each of thereceive zones 32, 34, 36, 38 and transmit zones 31, 33, 35, 37,employing the point-to-point service utilizes the entire frequencyspectrum (i.e. 500 MHz). Furthermore, this total frequency spectrum isdivided into a plurality of channels, for example, sixteen channels eachhaving a usable bandwidth of 27 MHz and a spacing of 30 MHz. In turn,each of the sixteen channels may accommodate approximately 800subchannels. Hence, within each zone, approximately 12,500 (16channels×800 subchannels) 32 kilobit per second channels may beaccommodated, at any given moment. As will be discussed below, thecommunication architecture of the point-to-point system allows anyterminal to communicate directly with any other terminal. Thus, within asingle polarization, a total of 50,000 subchannels may be accommodatednationwide.

Referring now particularly to FIGS. 1, 2, 6, 7 and 16, thepoint-to-point receive feed array 16 employs seven receive horns 50-62.Horns 50, 54, 58 and 62 respectively receive signals from zones 32, 34,36 and 38. Horns 52, 56 and 60 receive signals from the zones ofcontention 40, 42 and 44. Using a series of hydrid couplers or powerdividers C₁ -C₉, the signals received by horns 50-62 are combined intofour outputs 64-70. For example, a signal originating from the area ofcontention 44 and received by horn 60 is divided by coupler C₂ andportions of the divided signal are respectively delivered to couplers C₁and coupler C₄ whereby the split signal is combined with the incomingsignals received by horns 58, 62 respectively. Similarly, signalsoriginating from the area of contention 42 and received by horn 56 aresplit by coupler C₅. A portion of the split signal is combined, bycoupler C₃, with the signal output of coupler C₄, while the remainingportion of the split signal is combined, by coupler C₇ , with the signalreceived by horn 54.

Attention is now particularly directed to FIG. 6 which depicts, in blockdiagram form, the electronics for receiving and transmitting signals forboth the CONUS and point-to-point systems. The point-to-point receivesignals 64-70 (see also FIG. 7) are derived from the point-to-pointreceive feed network in FIG. 7, whereas the CONUS receive signal 72derives from the CONUS receive feed horns 14, (FIGS. 1 and 3). Both thepoint-to-point and CONUS receive signal are input to a switching network76 which selectively connects input lines 64-72 with five correspondingreceivers, eight of which receivers are generally indicated at 74. Thereceivers 74 are of conventional design, three of which are provided forredundancy and are not normally used unless a malfunction in one of thereceivers is experienced. In the event of a malfunction, switchingnetwork 76 reconnects the appropriate incoming line 64-72 with a back-upreceiver 74. Receivers 74 function to drive the filters in a filterinterconnection matrix 90. The outputs of the receivers 74, which areconnected with lines 64-70, are coupled by a second switching network 78through four receiver lines R1-R4 to a filter interconnection matrix 90.As will be discussed later below, the filter interconnection matrix(FIM) provides interconnections between the receive zones 32, 34, 36,38, and the transmit zones 31, 33, 35, 37. Operating in theabove-mentioned 500 MHz assigned frequency spectrum, separated intosixteen 27 MHz channels, four sets of sixteen filters are employed. Eachset of the sixteen filters utilizes the entire 500 MHz frequencyspectrum and each filter has a 27 MHz bandwidth. As will be discussedlater, the filter outputs T1-T4 are arranged in four groups, each groupdestined for one of the four transmit zones 31, 33, 35, 37.

The transmit signals T1-T4 are respectively connected, via switchingnetwork 94, to four of six driving amplifiers 92, two of such amplifiers92 being provided for back-up in the event of failure. In the event ofthe failure of one of the amplifiers 92, one of the back-up amplifiers92 will be reconnected to the corresponding transmit signal T1-T4 by theswitching network 94. A similar switching network 96 couples theamplified output of the amplifiers 92 to a beam-forming network 98. Aswill be discussed later in more detail, the beam-forming network 98consists of a plurality of transmission delay lines connected at equalintervals along the four delay lines. These intervals and the width ofthe delay lines are chosen to provide the desired centerband beam squintand the beam scan rate with frequency for the corresponding transmitzones 31, 33, 35, 37 to be serviced. The transmit signals, coupled fromthe four delay lines, are summed in the beam-forming network 98 as shownin FIGS. 11 and 12, to provide inputs to solid state power amplifiers100, which may be embedded in the point-to-point system's transmit array20. In the illustrated embodiment discussed below, forty solid statepower amplifiers (SSPAs) 100 are provided. Each of the SSPAs 100amplifies a corresponding one of the forty signals formed by thebeam-forming network 98. The SSPAs 100 possess different powercapacities to provide the tapered array excitation previously mentioned.The output of the SSPA 100 is connected to the input 112 (FIG. 14) atone of the elements of the transmit array 20.

The receive signal for CONUS on line 72 is connected to an appropriatereceiver 74 by switching networks 76, 78. The output of the receiverconnected with the CONUS signal is delivered to an input multiplexer 80which provides for eight channels, as mentioned above. The purpose ofthe input multiplexers 80 is to divide the one low level CONUS signalinto subsignals so that the subsignals can be amplified on an individualbasis. The CONUS receive signals are highly amplified so that the CONUStransmit signal may be distributed to very small earth terminals. Theoutputs of the input multiplexer 80 are connected through a switchingnetwork 84 to eight of twelve high power traveling wave tube amplifiers(TWTAs) 82, four of which TWTAs 82 are employed for backup in the eventof failure. The outputs of the eight TWTAs 82 are connected throughanother switching network 86 to an output multiplexer 88 whichrecombines the eight amplifed signals to form one CONUS transmit signal.The output of the multiplexer 88 is delivered via waveguide to thetransmit horns of the CONUS transmitter 24 (FIGS. 2 and 3).

Attention is now directed to FIG. 10 which depicts the details of theFIM 90 (FIG. 6). As previously discussed, the FIM 90 effectivelyinterconnects any terminal in any of the receive zones 32, 34, 36, 38(FIG. 5) with any terminal in any of the transmit zones 31, 33, 35, 37.The FIM 90 includes four waveguide inputs 120, 122, 124 and 126 forrespectively receiving the receive signals R1, R2, R3 and R4. Aspreviously mentioned, receive signals R1-R4, which originate from acorresponding receive zone 32, 34, 36, 38 (Fig. 5), each contain theentire assigned frequency spectrum, (e.g. 500 MHz), and are separatedinto a plurality of channels, (e.g. sixteen 27 MHz channels). Thechannels are further separated into a plurality of subchannels, whereeach of the subchannels carries a signal from a corresponding uplinksite. The FIM 90 includes 64 filters, one of which is indicated by thenumeral 102. Each of the filters 102 has a passband corresponding to oneof the channels (e.g. 1403-1430 MHz). The filters 102 are arranged infour groups, one for each receive zone 32, 34, 36, 38, with each groupincluding two banks or subgroups of eight filters per subgroup. Onesubgroup of filters 102 contains those filters for the even-numberedchannels and the other subgroup in each group contains eight filters forthe odd- numbered channels. Thus, for example, the filter group forreceive signal R1 comprises subgroup 104 of filters 102 for oddchannels, and subgroup 106 of filters 102 for even channels. Thefollowing table relates the receive signals and zones to their filtersubgroups:

    ______________________________________                                                       Filter Subgroups                                               Receive Zone                                                                           Receive Signal                                                                            Odd Channels                                                                              Even Channels                                ______________________________________                                        32       R1          104         106                                          34       R2          108         110                                          36       R3          112         114                                          38       R4          116         118                                          ______________________________________                                    

The filters are grouped in a unique manner such that when the receivesignals R1-R4 are filtered, the filtered outputs are combined to formthe transmit signals. The transmit signals T1-T4 also utilize the entireassigned frequency spectrum, (e.g. 500 MHz). In the illustratedembodiment, each of the transmit signals T1-T4 possesses sixteen 27 MHzwide channels, and comprises four channels from each of the four receivezones 32-38 (FIG. 5).

The incoming receive signals R1-R4 are divided into the correspondingsubgroups by respectively associated hybrid couplers 128-134 whicheffectively divert 50% of the signal power to each subgroup. Hence, forexample, one-half of the R1 signal input at waveguide 120 is diverted totransmission line 136 which services the subgroup 104 of filters 102,and the remaining half of the R1 signal is diverted to transmission line138 which services subgroup 106 of filters 102. In a similar manner,each of the subgroups 104-118 of filters 102 is served by acorresponding distribution line, similar to lines 136 and 138.

The construction of subgroup 104 will now be described in more detail,it being understood that the remaining subgroups 106-118 are identicalin architecture to subgroup 104. At intervals along the transmissionline 136, there are eight ferrite circulators 140, one associated witheach of the odd-numbered channel filters 102. The function of thecirculators 140 is to connect the transmission line 136 to each of theodd channel filters 102 in a lossless manner. Thus, for example, the R1signal enters the first circulator 140a and circulates itcounterclockwise whereby the 27 MHz band of signals corresponding tochannel 1 passes through it to circulator 142. All other frequencies arereflected. These reflected signals propagate via the circulator towardthe next filter where the process is repeated. Through this process, theR1 receive signal is filtered into sixteen channels by the sixteenfilters 104-108 corresponding to the R1 signals. Hence, the R1 signalwith frequencies in the range of channel 1 will pass through the firstferrite circulator 140a and it will be filtered by filter 1 of group104.

The outputs from a filter subgroup 104-118 are selectively coupled by asecond set of ferrite circulators 142 which sums, in a criss-crosspattern, the outputs from an adjacent group of filters 102. For example,the outputs of channel filters 1, 5, 9, and 13 of group 104 are summedwith the outputs of channel filters 3, 7, 11 and 15 of filter group 112.This sum appears at the output terminal for T1 144. Referring to FIG. 8,these signals correspond to the connections between receive zones R1 andR3 and to transmit zone T1.

Attention is now directed to FIGS. 8 and 9 which depict how the transmitand receive signals are interconnected by the FIM 90 to allow two-waycommunication between any terminals. Specifically, FIG. 8 provides atable showing how the receive and transmit zones are connected togetherby the interconnect channels while FIG. 9 depicts how these interconnectchannels are distributed geographically across the transmit zones 31,33, 35, 37. In FIG. 8, the receive signals R1-R4 are read across by rowsof interconnect channels and the transmit signals T1-T4 are read bycolumns of interconnect channels. It can be readily appreciated fromFIG. 8 that each of the transmit signals T1-T4 is made up of sixteenchannels arranged in four groups respectively, where each group isassociated with one of the receive signals R1-R4. The satellitecommunications system of the present invention is intended to be used inconjunction with a ground station referred to as a satellite networkcontrol center which coordinates communications between the groundterminals via packet switched signals. The network control centerassigns an uplink user with an uplink frequency based on the location ofthe desired downlink, assigning the available frequency whose downlinklongitude is closest to that of the destination. The frequencyaddressable downlink transmit beams 29 are thus addressable by thefrequencies of the uplink signals. This strategy maximizes the gain ofthe downlink signal.

As shown in FIG. 9, the continental United States is divided into fourprimary zones 31, 33, 35, 37. Zone 31 may be referred to as the EastCoast zone, zone 33 is the Central zone, zone 35 is the Mountain zone,and zone 37 is the West Coast zone. As previously mentioned, each of thezones 31, 33, 35, 37 utilizes the entire assigned frequency spectrum(e.g. 500 MHz). Thus, in the case of a 500 MHz assigned frequency band,there exists sixteen 27 MHz channels plus guard bands in each of thezones 31, 33, 35, 37.

The numbers 1-16 repeated four times above the beams 29 in FIG. 9indicate the longitude of the beams corresponding to the centerfrequencies of the channels so numbered. Because of the frequencysensitivity of the beams, the longitude span between the lowest andhighest frequency narrow band signal in a channel is approximately onechannel width. Each beam is 0.6 degrees wide between its half powerpoint, about half the zone width in the East Coast and Central zones andnearly one-third the zone width in the Mountain and West Coast zones.The antenna beams 29 overlap each other to ensure a high signal density;the more that the beams overlap, the greater channel capacity in a givenarea. Hence, in the East Coast zone 31, there is a greater overlap thanin the Mountain zone 35 because the signal traffic in the East Coastzone 31 is considerably greater than that in the Mountain zone 35.

The interconnect scheme described above will now be explained by way ofa typical communication between terminals in different zones. In thisexample, it will be assumed that a caller in Detroit, Mich. wishes toplace a call to a terminal in Los Angeles, Calif. Thus, Detroit, Mich.,which is located in the Central zone 34, is the uplink site, and LosAngeles, Calif., which is located in the West Coast zone 37, is thedownlink destination. As shown in FIG. 9, each geographic location inthe continental United States can be associated with a specific channelin a specific zone. Thus, Los Angeles is positioned between channels 14and 15 in transmit zone 37.

Referring now concurrently to FIGS. 5, 8 and 9 particularly, receive andtransmit zones R1 and T1 lie within the East Coast zone 32 and 31, R2and T2 lie within the Central zone 34 and 33, R3 and T3 lie within theMountain zone 36 and 35, and R4 and T4 lie within the West Coast zone 38and 37. Since Detroit lies in the Central or R2 zone 34, it can be seenthat the only channels over which signals can be transmitted to the WestCoast or T4 zone 37 are channels 1, 5, 9 and 13. This is determined inthe table of FIG. 8 by the intersection of row R2 and column T4.Therefore, from Detroit, the uplink user would uplink on either channel1, 5, 9 or 13, whichever of these channels is closest to the downlinkdestination. Since Los Angeles is located between channels 14 and 15,the network control center would uplink the signal on channel 13 becausechannel 13 is the closest to channel 14. The downlink beam width isbroad enough to provide high gain at Los Angeles.

Conversely, if the uplink site is in Los Angeles and the downlinkdestination is in Detroit, the intersection of row R4 and column T2 inFIG. 8 must be consulted. This intersection reveals that the signal canbe transmitted on channels 1, 5, 9 or 13 depending upon which channel isclosest to the downlink destination. The network control center woulduplink the signal from Los Angeles on channel 9 since channel 9 isclosest to channel 11 which, in turn, is closest to Detroit.

Returning now to FIG. 10, the conversion on a receive signal to atransmit signal will be described in connection with the examplementioned above in which the uplink site is in Detroit and the downlinksite is in Los Angeles. The uplink signal transmitted from Detroit wouldbe transmitted on channel 13 carried by receive signal R2. Thus, the R2receive signal is input to transmission line 122 and a portion of suchinput signal is diverted by the hybrid coupler 130 to the input line ofsubgroup 108 of filters 102. Subgroup 108 includes a bank of eightfilters for the odd channels, including channel 13. Thus, the incomingsignal is filtered through by filter 13 and is output on a line 164along with other signals from subgroups 108 and 116. The channel 13signal present on line 164, is combined by the hybrid coupler 158, withsignals emanating from subgroup 106 and 114, and forms the T4 signal onoutput line 150. The transmit signal T4 is then downlinked to LosAngeles.

It is to be understood that the above example is somewhat simplifiedinasmuch as the network control center would assign a more specificchannel than a 27 MHz wide band channel, since the 27 MHz wide channelmay actually comprise a multiplicity of smaller channels, for example,800 subchannels of 32 KHz bandwidth.

Referring now again to FIGS. 5, 8 and 9, in the event that an uplinksignal originates from one of the areas of contention, 40, 42, 44 (FIG.5), such signal will not only be transmitted to its desired downlinkdestination, but a non-neglible signal will be transmitted to anothergeographic area. For example, assume that the uplink signal originatesfrom Chicago, Ill. which is in the area of contention 42 and that thesignal is destined for Los Angeles, Calif. The area of contention 42 isproduced by the overlap of the beams forming zones 34 and 36. Hence, theuplink signal can be transmitted as receive signals R2 or R3. Thenetwork control center determines whether the uplink communication iscarried by receive signals R2 or R3. In the present example, sinceChicago is closer to zone 36, the uplink communication is carried onreceive signal R3.

As previously discussed, the downlink destination, Los Angeles, islocated in zone 37 and lies between channels 14 and 15. As shown in FIG.8, the intersection of R3 with column T4 yields the possible channelsover which the communication can be routed. Thus, the Chicago uplinksignal will be transmitted over one of channels 2, 6, 10 or 14. SinceLos Angeles is closest to channel 14, channel 14 is selected by thenetwork control center as the uplink channel. Note, however, that anundesired signal is also transmitted from zone 34 on channel 14. Todetermine where the undesired signal will be downlinked, the table ofFIG. 8 is consulted. The table of FIG. 8 reveals that uplink signalscarried on channel 14 in the R2 zone 34 are downlinked to the T1transmit zone 31. The desired signal is transmitted to Los Angeles andthe undesired signal (i.e. an extraneous signal) is transmitted to theEast Coast (i.e. zone 31). The network control center keeps track ofthese extraneous signals when making frequency assignments. The effectof these extraneous signals is to reduce slightly the capacity of thesystem.

Referring now again to FIG. 6, the beam-forming network 98 receives thetransmit signals T1-T4 and functions to couple all of the individualcommunication signals in these transmit signals together so that atransmit antenna beam for each signal is formed. In the examplediscussed above in which the assigned frequency spectrum is 500 MHz, atotal of approximately 50,000 overlapping antenna beams are formed bythe beam-forming network 98 when the system is fully loaded with narrowband signals. Each antenna beam is formed in a manner so that it can bepointed in a direction which optimizes the performance of the system.The incremental phase shift between adjacent elements determines thedirection of the antenna beam. Since this phase shift is determined bythe signal frequency, the system is referred to as frequency addressed.

Attention is now directed to FIGS. 11 and 12 which depict the details ofthe beam-forming network 98. The beam-forming network, generallyindicated by the numeral 98 in FIG. 11, is arranged in the general formof an arc and may be conveniently mounted on the communication shelf(not shown) of the satellite. The arc shape of the beam-forming network98 facilitates an arrangement which assures that the paths of thesignals passing therethrough are of correct length.

The beam-forming network 98 includes a first set of circumferentiallyextending transmission delay lines 168, 170, a second set oftransmission delay lines 172, 174 which are radially spaced from delaylines 168 and 170, and a plurality of radially extending waveguideassemblies 176. In the illustrated embodiment, forty waveguideassemblies 176 are provided, one for each of the elements 106 of thetransmit array 20 (FIG. 13). The waveguide assemblies 176 intersect eachof the delay lines 168-174 and are equally spaced in angle.

Each of the waveguide assemblies 176 defines a radial line summer andintersects each of the delay lines 168-174. As shown in FIG. 12, at thepoints of intersection, between the radial line summers 176 and thetransmission delay lines 168-174, a crossguide coupler 180 is provided.The crossguide coupler 180 connects the delay lines 168-174 with theradial line summers 176. The function of the crossguide couplers 180will be discussed later in more detail.

Four delay lines 168-174 are provided respectively for the four transmitzones T1-T4 (FIG. 9). Hence, transmit signal T1 is provided to the inputof delay line 170, T2 is provided to input of delay line 168, T3 isprovided to the input of delay line 174, and T4 is provided to the inputof delay line 172. As shown in FIG. 12, the distance between the radialline summers is indicated by the letter "l" and the width of each of theradial delay lines is designated by the letter "w". Although the radialline summers 176 are spaced at equal angular intervals along the delaylines 168-174, the distance between them varies from delay line to delayline due to the fact that the delay lines 168-174 are radially spacedfrom each other. Thus, the further from the center of the arc, which isformed by the radial line summers 176, the greater the distance betweenthe radial line summers 176, at the point where they intersect with thedelay lines 168-174. In other words, the spacing "l" between radial linesummers 176 for delay line 168 is less than the spacing between adjacentradial line summers 176 than for delay line 174. Typical values for thedimensions "l" and "w" are as follows:

    ______________________________________                                        Delay Line                                                                              Signal       l, inches                                                                              w, inches                                     ______________________________________                                        168       T2           1.66     0.64                                          170       T1           1.72     0.66                                          172       T4           2.45     0.74                                          174       T3           2.55     0.76                                          ______________________________________                                    

The width of the delay lines 168-174, "w", and the distance "l" betweenadjacent radial line summers are chosen to provide the desired centerbeam squint and beam scan rate so that the beam pointing is correct foreach channel. This results in the desired start and stop points for eachof the transmit zones T1-T4.

Referring particularly to FIG. 12, the transmit signal T2 propagatesdown the delay line 168 for a precise distance, at which point itreaches the first radial line summer 176. A portion of the T2 signalpasses through the crossguide coupler 180, which may, for example, be a20 dB coupler, such that one percent of the transmitted power oftransmit signal T2 is diverted down the radial line summer 176. Thisdiverted energy then propagates down the waveguide 176 towards acorresponding solid state power amplifier 100 (FIGS. 6 and 11). Thisprocess is repeated for signal T1 which propagates down delay line 170.The portions of signals T1 and T2 which are diverted by the crossguidecoupler 180 (i.e. 0.01 T1 and 0.01 T2) are summed together in the radialline summer 176 and the combined signal 0.01 (T1+T2) propagates radiallyoutwardly toward the next set of delay lines 172, 174. This samecoupling process is repeated for signals T3 and T4 in delay lines 174and 172 respectively. That is, 0.01 of signals T3 and T4 are coupled viacrossguide couplers 180 to the radial line summer 176. The resultingcombined signal 0.01 (T1+T2+T3+T4) propagates radially outwardly to anassociated solid state power amplifier 100 where it is amplified inpreparation for transmission.

After encountering the first radial line summer 176, the remaining 0.99of signals T1-T4 propagate to the second radial line summer where anadditional one percent of the signals is diverted to the summer 176.This process of diverting one percent of the signals T1-T4 is repeatedfor each of the radial line summers 176.

The signals, propagating through the radial line summers 176 towards theSSPAs 100, are a mixture of all four point-to-point transmit signalsT1-T4. However, each of the transmit signals T1-T4 may comprise 12,500subsignals. Consequently, the forty signals propagating through theradial line summers 176 may be a mixture of all 50,000 signals in thecase of the embodiment mentioned above where the assigned frequencyspectrum is 500 MHz wide. Therefore, each of the SSPAs 100 amplifies all50,000 signals which emanate from each of the plurality of wave guideassemblies 176.

Since each of the SSPAs 100 amplifies all 50,000 signals which aredestined for all regions of the country, it can be appreciated that allof the narrow, high gain downlink beams are formed from a common pool oftransmitters, i.e. all of the SSPAs 100. This arrangement may be thoughtof as effectively providing a nationwide pool of power since each of thedownlink beams covering the entire country is produced using all of theSSPAs 100. Consequently, it is possible to divert a portion of thisnationwide pool of power to accommodate specific, disadvantaged downlinkusers on an individual basis without materially reducing the signalpower of the other beams. For example, a downlink user may be"disadvantaged" by rain in the downlink destination which attenuates thesignal strength of the beam. Such a rain disadvantaged user may beindividually accommodated by increasing the signal strength of thecorresponding uplink beam. This is accomplished by diverting to thedisadvantaged downlink beam, a small portion of the power from the poolof nationwide transmitter power (i.e. a fraction of the power suppliedby all of the SSPAs 100). The power of an individual uplink beam isproportional to that of the corresponding downlink beam. Consequently,in order to increase the power of the downlink beam it is merelynecessary to increase the power of the uplink beam.

In practice, the previously mentioned network control center keeps trackof all of those regions of the country in which it is raining anddetermines which of the uplink users are placing communications todownlink destination in rain affected areas. The network control centerthen instructs each of these uplink users, using packet switchedsignals, to increase its uplink power for those signals destined for arain affected area. The increase in power of the uplink user's signalsresults in greater collective amplification of these signals by theSSPAs 100, to produce corresponding downlink beams to the rain affectedareas, which have power levels increased sufficiently to compensate forrain attenuation. Typically, the number of signals destined for rainaffected areas is small relative to the total number of signals beinghandled by the total pool of SSPAs 100. Accordingly, other downlinkusers not in the rain affected zones do not suffer substantial signalloss since the small loss that may occur in their signals is spread outover the many thousand users.

The SSPAs 100 (FIGS. 8 and 11) may be mounted, for example, on the rimof the communication shelf (not shown) of the satellite. The signalsamplified by the SSPAs 100 are fed into the corresponding elements 106of the transmit array 20 (FIGS. 13 and 14).

As previously discussed, an incremental phase shift is achieved betweenthe signals that are coupled in the forty radial line summers 176.Hence, the beam-forming network 98 permits the antenna beams emanatingfrom the transmit array 20 (FIGS. 1, 2, and 13) to be steered byfrequency assignment. The incremental phase shift is related to the timedelay between the waveguides 176 as well as frequency. Attention is nowdirected to FIG. 17 which is a diagrammatic view of four of the fortytransmit array elements 106 (FIG. 13), showing the wavefront emanatingtherefrom, wherein "d" is equal to the spacing between transmit arrayelements 106. The resulting antenna beam has an angular tilt of θ, whereθ is defined as the beam scan angle. This means that θ is the angle fromnormal of the transmit beam center. The incremental phase shift producedby the delay line arrangement is ΔΦ. The relationship between ΔΦ and θis given by ##EQU1## where: λ=signal wavelength

θ=beam scan angle

d=spacing between array elements

Hence, the east-west direction of the antenna beam is determined by theincremental phase shift which is different for the four delay lines168-174 of the beam-forming network 98, resulting in the four transmitzones T1-T4 previously noted.

Having thus described the invention, it is recognized that those skilledin the art may make various modifications or additions to the preferredembodiment chosen to illustrate the invention without departing from thespirit and scope of the present contribution to the art. Accordingly, itis to be understood that the protection sought and to be afforded herebyshould be deemed to extend to the subject matter claimed and allequivalents thereof fairly within the scope of the invention.

What is claimed is:
 1. Apparatus for communicatively interconnecting anyof a plurality of terminal sites within an area on the earth,comprising:a satellite positioned above the earth; means carried by saidsatellite for respectively receiving from uplink terminal sites in saidarea a plurality of uplink radio frequency beams each carrying a receivesignal; means carried by said satellite for converting said receivesignals into transmit signals each including a plurality ofcorresponding subsignals destined to be received at downlink terminalsites in said area by changing the frequencies of said receive signals;a plurality of amplifiers carried by said satellite for collectivelyamplifying all of said transmit signals such that each of said transmitsignals is amplified by all of said amplifiers; and means carried bysaid satellite for forming a plurality of downlink radio frequency beamseach carrying one of said transmit subsignals destined to be received byone of said downlink terminal sites and primarily covering only aportion of said area, said beam-forming means including - (1) a firstplurality of lines for respectively carrying said transmit signals, and(2) a second plurality of spaced apart lines intersecting said firstplurality of lines at crossover points and extending radially from areference point so as to diverge from each other, each of said secondplurality of lines being coupled with each of said first plurality oflines at said crossover points such that a portion of the energy of eachof the transmit signals carried by each of the first plurality of linesis transferred to each of said second plurality of lines, each of saidsecond plurality of lines having an output for outputting all of saidtransmit signals, each of said outputs being associated with and coupledto one of said amplifiers such that each of said amplifiers amplifiesall the transmit signals from an associated one of said amplifiers; andmeans carried by said satellite for respectively transmitting saiddownlink beams to corresponding portions of said area.
 2. The apparatusof claim 1, wherein said first plurality of lines extendcircumferentially about said reference point and are radially spacedrelative to said reference point such that the distance between adjacentcrossover points increases with increasing radial distance of thecrossover points from said reference point.
 3. The apparatus of claim 2,wherein at least certain of the lines in said first plurality of linesare unequally radially spaced apart from each other.
 4. The apparatus ofclaim 2, wherein said first plurality of lines includes a first set ofessentially contiguous lines and a second set of essentially contiguouslines spaced apart from said first set thereof.
 5. Apparatus forcommunicatively interconnecting any of a plurality of terminal siteswithin an area on the earth, comprising:a satellite positioned above theearth; means carried by said satellite for respectively receiving fromuplink terminal sites in said area a plurality of uplink radio frequencybeams each carrying a receive signal; means carried by said satellitefor converting said receive signals into transmit signals each includinga plurality of corresponding subsignals destined to be received atdownlink terminal sites in said area by changing the frequencies of saidreceive signals; a plurality of amplifiers carried by said satellite forcollectively amplifying all of said transmit signals such that each ofsaid transmit signals is amplified by all of said amplifiers; and meanscarried by said satellite for forming a plurality of downlink radiofrequency beams each carrying one of said transmit subsignals destinedto be received by one of said downlink terminal sites and primarilycovering only a portion of said area, said beam-forming meansincluding - (1) a first plurality of lines for respectively carryingsaid transmit signals, and (2) a second plurality of spaced apart linesintersecting said first plurality of lines at crossover points andextending radially from a reference point so as to diverge from eachother, each of said second plurality of lines being coupled with each ofsaid first plurality of lines at said crossover points such that aportion of the energy of each of the transmit signals carried by each ofthe first plurality of lines is transferred to each of said secondplurality of lines, each of said second plurality of lines having anoutput for outputting all of said transmit signals, each of said outputsbeing associated with and coupled to one of said amplifiers such thateach of said amplifiers amplifies all the transmit signals from anassociated one of said amplifiers, said first plurality of linesextending circumferentially about a reference point and being radiallyspaced relative to said reference point such that the distance betweenadjacent crossover points increases with increasing radial distance ofthe crossover points from said reference point; and means carried bysaid satellite for respectively transmitting said downlink beams tocorresponding portions of said area.
 6. The apparatus of claim 5,wherein at least certain of the lines in said first plurality of linesare unequally radially spaced apart from each other.
 7. The apparatus ofclaim 5, wherein said first plurality of lines includes a first set ofessentially contiguous lines and a second set of essentially contiguouslines spaced apart from said first set thereof.